Skid control system for vehicles

ABSTRACT

In a skid control system a pressure modulator reduces the pressure applied by a master cylinder to a wheel cylinder when the wheel velocity as measured by a wheel detector decreases faster than a deceleration predetermined by a deceleration control. A control unit which regulates the pressure modulator corrects the predetermined deceleration when the wheel has exceeded the slip ratio at which the coefficient of friction is maximum so as to achieve an optimum deceleration that corresponds to the road surface condition. The control unit induces the pressure modulator to reduce the pressure of the brake fluid and restore the wheel velocity when the coefficient of friction of the wheel reaches the maximum. Emergency braking is thus performed with an average slip ratio at which the coefficient of friction is maximum. A comparator circuit compares the wheel velocity with a signal from the control unit indicative of the velocity of the predetermined decelerator to determine when the wheel velocity has decreased faster than the predetermined deceleration.

This is a continuation of application Ser. No. 510,032 filed Sept. 27,1974, now abandoned, which is a division of application Ser. No. 270,584filed July 11, 1972, now U.S. Pat. No. 3,848,933, which in turn was acontinuation-in-part application of Ser. No. 109,465 and Ser. No.109,461, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates to skid control systems for the brakes ofvehicles, such as automobiles and the like; and particularly to systemswhich prevent an operator from losing control of a vehicle, due tolocking of the wheels, by regulating the effectiveness of the brakes independence upon the road surface conditions, and regardless of the pedaleffort, during an emergency stop.

The invention has particular, although not exclusive reference, tosystems for regulating the pressure of brake fluids applied to the wheelcylinders of a brake.

A skid control system may be composed of a wheel velocity detector, acontrol unit and a pressure modulator for modulating the pressure ofbrake fluid. In such a skid control system, a deceleration is preset asa desired limit on the basis of road surface conditions which areindirectly detected from factors, such as changes in the wheel velocity.The control unit compares the preset deceleration with the actual wheeldeceleration. The latter is derived from the wheel velocity as detectedby the wheel velocity detector. The comparator issues pressure reductionsignals to the pressure modulator when the wheel deceleration exceedsthe preset deceleration. The pressure modulator responds by reducing thepressure of the brake fluid.

Known methods for determining the actual wheel deceleration employ adifferentiation circuit composed of resistors and capacitors. Thedifferential value of wheel velocity is then used as the wheeldeceleration.

It is difficult to obtain accurate indications of actual wheeldecelerations by means of differentiation circuits, because at the lowfrequency at which the velocity varies, namely on the order of severalHertz, it is difficult to obtain exact differential values. Moreover,the differential signal obtained is subject to distortion by noise. Thisresults in skid control systems with unstable performance.

Also, skid control systems are required to perform their brakingfunctions so as to achieve minimum stopping distances while preventingthe wheels from locking. Under such circumstances it is desirable thatbraking be accomplished when the coefficient of friction between thewheels and the road surface is maximum. Hitherto it has been assumedthat the point of maximum coefficient of friction is usually in therange of 0.15 - 0.2 of the wheel slip ratio. Consequently, systems havebeen used wherein a constant slip ratio control falling within thisrange is set and utilized to regulate a pressure modulator. In suchsystems, if the slip ratio at which the coefficient of friction ismaximum, varies because of changes in the road surface or because ofchanges in the condition of the road surface or because of changes inthe shapes of tires or for any number of reasons, skid control isreduced.

An object of this invention is to improve skid control systems.

Another object of this invention is to obviate the disadvantages ofprevious skid control systems.

Still another object of the invention is to render skid control systemsresponsive to a number of variables, such as the kind of road surfaceand its condition.

Yet another object of the invention is to adjust the skid control systemcontinuously for various road conditions and other variables, yetanother feature of the invention is to adjust skid control systems in amanner most appropriate for braking a vehicle with a slip ratio havingthe maximum coefficient of friction.

SUMMARY OF THE INVENTION

According to a feature of the invention, the deficiencies of prior artdeceleration controls and skid control systems are obviated by circuitmeans which respond to the wheel velocity for forming the control signalcorresponding to the velocity sensed, regulator means which establish anindication representative of a predetermined limit to the deceleration,and network means responsive to the regulator means and coupled to thecircuit means which inhibit the changes in the value of the controlsignal formed by the circuit means when the changes occur at a rategreater than the indication. Electrical means respond to the controlsignal to produce an output when the change in the value of the controlsignal is inhibited.

According to another feature of the invention, control means respond tothe electrical means to modulate the braking effect of a vehicle'sbraking system and sensing means sense the speed of a wheel of avehicle.

According to another feature of the invention, the network meansincludes a capacitor responsive to the control signal of the circuitmeans. It also includes a current flow control device, such as atransistor current amplifier, responsive to the indication of the presetdeceleration, which controls the rate of discharge of the capacitor.When the rate of discharge of the capacitor no longer follows the actualvelocity, the electrical means detect this unbalance and cause thecontrol means to modulate the braking effect.

According to another feature of the invention, the electrical meansconstitutes a differential comparator circuit.

According to another feature of the invention, the braking force on thewheel of a vehicle is adjusted so that it decreases when the wheelvelocity decreases faster than the predetermined limit and by adjustingthe rate on the basis of changes of wheel velocity.

According to another feature of the invention, the predetermineddeceleration limit is adjusted downwardly on the basis of how long thewheel velocity requires to start decreasing after the brake effort hasincreased in response to a wheel velocity increase.

According to another feature of the invention, the predetermineddeceleration limit is readjusted on the basis of the time required forthe rate of the decrease in wheel velocity to surpass the setdeceleration.

According to still another feature of the invention, control meansrespond to the velocity of the wheel for producing a signal whe thewheel velocity decreases faster than a given deceleration limit, brakemeans decrease the braking effect of the system in response to thesignal and reverse the decrease in the absence of the signal, velocitychange responsive means in the control means produce a first indicationin response to increases in the wheel velocity and a second indicationin response to decreases in the wheel velocity, during the absence ofthe signal, and setting means respond to the control means for adjustingthe given deceleration limit downwardly in response to first indicationand upwardly in response to the second indication.

According to yet another feature of the invention, regulating meansrespond to the setting means for decreasing the braking effect of thesystem during the first indication, even in the absence of the signal.

According to still another feature of the invention, maintainance meanscause the brake system to maintain the braking effect at the onset ofthe first indication and throughout the first indication.

According to yet another feature of the invention, the brake modulationmeans vary the brake fluid pressure to a brake cylinder from a mastercylinder.

The invention is based upon the recognition that if the slip ratio ofthe wheels exceed the slip ratio that occurs at the maximum coefficientof friction, when the pressure of the brake fluid is increased linearly,the deceleration increases rapidly and therefore the wheel velocitydecreases rapidly. In effect the invention detects the time when thewheel deceleration increases rapidly.

These and other features of the invention are pointed out in the claims.Other objects and advantages of the invention will become apparent tothose skilled in the art from the following detailed description, whenread in light of the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing illustrating a skid control system mountedin a brake line according to the prior art;

FIG. 2 is a schematic diagram illustrating a skid control systemembodying features of the invention;

FIG. 3 is a characteristic diagram illustrating the variations involtages at various times for variation in pressures shown in FIG. 4;

FIG. 4 is a diagram illustrating the change in pressure of the brakefluid which creates the voltages in FIG. 3;

FIG. 5 is a schematic circuit diagram illustrating another embodiment ofa portion of the circuit in FIG. 1.

FIG. 6 is a schematic drawing illustrating a brake arrangement with askid control system each embodying features of the invention.

FIG. 7 is a graph illustrating various characteristics of coefficientsof friction for wheel slip ratios in vehicles, such as those in FIG. 6;

FIG. 8a is a graph illustrating changes in wheel velocity as compared tothe vehicle velocity in a vehicle, such as that illustrated in FIG. 6,as these velocities change with respect to time;

FIG. 8b is a presure time diagram illustrating the change in fluidpressure in the master cylinder of FIG. 6;

FIG. 9 is a schematic diagram illustrating the control unit and othermembers embodying features of the invention and forming a part of thesystem in FIG. 1;

FIGS. 10a-10g are graphs illustrating various velocities and voltages ofportions of the system in FIGS. 6 and 9;

FIG. 11 is a schematic diagram illustrating another embodiment of thecontrol unit in FIGS. 6 and 9 but showing only those portions of thecontrol unit that differentiates this control unit from the control unitin FIG. 9;

FIG. 12 is a schematic diagram of still another embodiment of thecontrol unit in FIG. 6;

FIG. 13 is a schematic diagram of a variation of the control unit inFIG. 12;

FIG. 14 is a schematic diagram illustrating still another variation ofthe embodiment of the control unit illustrated in FIG. 12;

FIG. 15 is a partly schematic and partly sectional diagram of thepressure modulator in FIGS. 6, 9, 12 and 14; and

FIG. 16 is a schematic diagram of another variation of the embodiment ofFIG. 12.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the prior art skid control system of FIG. 1, a brake pedal 1, whendepressed, actuates a master cylinder 2. A control unit 3 electricallycontrols a pressure modulator 4. The latter, in response to the controlunit 3, modulates the pressure which the master cylinder 2 applies to awheel cylinder 5 of a wheel 7 whose velocity and acceleration is to becontrolled. A wheel velocity or speed of wheel 7 is electricallyconnected to the control unit 3.

Under normal travel conditions, where the skid control system is notrequired to operate, the pressure of brake fluid derived from the mastercylinder 2 and conforming to the physical effort applied to the brakepedal 1, is applied in unaltered condition to the wheel cylinder 5. Thisprovides braking force to the wheel. If there is danger of the wheellocking during an emergency stop, the control unit 3 of the skid controlsystem issues a pressure reduction signal. Upon receipt of this signal,the pressure modulator decreases or shuts off the flow of brake fluidfrom the master cylinder. This regulates the pressure of brake fluid inthe wheel cylinder. In a control unit 3, the actual wheel decelerationis measured by a differential circuit 9 and compared by a comparatorcircuit 10 with the output of a set deceleration generator 11. Thelatter establishes a signal corresponding to a desired maximumdeceleration α₀. The term "maximum deceleration" for the deceleration αis used to indicate that it represents a desired limit for vehicledeceleration based on prevailing road conditions and the like. When theactual deceleration α₁ as measured by the differentiator 9, exceeds thedeceleration α₀, the comparator 10 actuates the pressure modulator. Thedifferentiator 9 is composed of a series capacitor C and shunt resistorR in the usual manner.

As expressed earlier, the problems of devices, such as those in FIG. 1,lie generally in the differentiation circuits, such as 9, which arebulky and inaccurate at the frequencies with which the wheel velocitieschange, namely a few Hertz. Their output signals are also subject todistortion by noise.

In FIG. 2, the skid control system which embodies features of theinvention has an overall operation comparable to that of FIG. 1.Specifically downward pressure on a pedal 20, corresponding to the pedal1, applies pressure to a wheel cylinder 22 by means of a master cylinder24 that operates through a presure modulator 26. The master cylinder 24and presure modulator 26 as well as the wheel cylinder 22 corresponds tothe comparable elements 2, 4 and 5 in FIG. 1. The wheel cylinder 22applies a braking effort to a wheel 28 that corresponds to the wheel 7.A wheel velocity detector 30 corresponding to the wheel velocitydetector 6 furnishes a wheel velocity signal to a control unit 32 thatregulates the pressure modulator 26. The latter then controls the fluidpressure from the master cylinder 24 to the brake cylinder 22, therebycontrolling the braking effect on the wheel 28 in dependence upon thewheel velocity detector 30.

Within the control unit 32 the bases of two transistors Tr1 and Tr2,having the same characteristics, receive a voltage proportional to thewheel speed from the wheel velocity generator 30 at their respectivebases. Added to the voltage of the wheel velocity genrator is a constantvoltage established across a Zener diode ZD by a resistor R₄ and abettery power source B. The junction of the resistor R₄ and the Zenerdiode ZD forms a constant voltage driving point to establish a stablereference voltage for the generator 30. A transistor Tr6 has a main pathof current flow that serves a load circuit in the emitter circuit oftransistor Tr1. Loading the emitter of the transisor Tr2 is a capacitorC₁. A charging resistor R₂ forms a current path from the collector oftransistor Tr₂ to the positive terminal of the battery power source B.The main current flow path of a transistor Tr5 forms a directly paralleldischarge path across capacitor C₁. The effect of the discharge path isdetermined by the voltage output of a preset deceleration generator 34corresponding to the set deceleration generator 11. The set decelerationgenerator 34 controls the transistor Tr5 as well as the transistor Tr6by applying a voltage g to their respective bases.

When a voltage V corresponding to the wheel velocity is applied by thewheel velocity generator 30 to the bases of transitors Tr1 and Tr2,their conduction causes voltages Vw and Vt to appear at the emitters oftransistors Tr1 and Tr2. These voltages are lower than the voltage V bythe base-emitter voltage V_(BE). The voltage of the emitter oftransistor Tr2 charges the capacitor C₁ to the voltage Vt and continuesto charge the capacitor to this voltage as the wheel velocity varies. Atthe same time the voltage g appears at the emitter-grounded transistorsTr5 and Tr6. The emitter-collector circuits of transistors Tr5 and Tr6are connected directly to the emitter of transistor Tr1 and across tocapacitor C₁, and behave as current amplifiers. The transistors Tr1 andTr2 behave as voltage amplifiers.

In its current amplifier operation, the transistor Tr5 produces acollector current flow proportional to the output voltage g of the setdeceleration generator. Thus, transistor Tr5 forms a circuit forfurnishing a bypass current proportional to the voltage g around thecapacitor C₁. The transistors Tr1 and Tr2 operate as voltage amplifiersof high mu factor. The transistor Tr2 acts as a voltage source to chargethe capacitor C₁ to a level Vt corresponding to the voltage V and thewheel velocity. This continues until the voltage V tries to cause thevoltage Vt at the emitter of transistor Tr2 and across the capacitor C₁to drop faster than the transistor Tr5 is capable of discharging thecapacitor C₁ because of the voltage g. At that point, the capacitor Cdischarges only at the rate established by the transistor Tr5. Thus thevoltage source formed by the transistor Tr2 exhists only as long a Vw isnot less than Vt.

This operation is illustrated by the curves of FIGS. 3 and 4. Here thecurves in FIG. 3 illustrate the variation of the voltage V correspondingto the wheel velocity as derived from the wheel velocity generator 30and Zener diode ZD. The voltages Vt and Ve caused by the voltage dropacross the base emitter junctions of the transistors Tr1 and Tr2 aresubstantially identical until a rapid drop in the value Vt is preventedby the limited discharge capability of the capacitor C₁ because of thetransistor Tr5. FIG. 4 illustrates the change in fluid pressure of themaster cylinder and the brake cylinder during various travellingconditions.

Under normal travel conditions, when the brake is not applied, the brakefluid pressure is substantially 0 until a time t₁. The velocity Vappearing at the output of the generator 30 and at the bases oftransistors Tr1 and Tr2 remains nearly constant between the times 0 andt₁. The voltages Vt and Vw also remain substantially constant or followthe value of the voltage V at a level less than the voltage V by a valueV_(BE). When a pressure is applied on the brake fluid and increaseslinearly from the time t₁, the wheel velocity voltage V graduallydecreases and the emitter voltages Vw and Vt of the transistors Tr1 andTr2 decrease correspondingly. As long as the voltage drop rate of thevoltage Vt is smaller than the voltage drop rate permitted in thecapacitor C₁ by the bypass current in the transistor Tr5, the transistorTr2 continues to determine the voltage at the junction of the capacitorC₁ and the emitter of transistor Tr2. That is to say the transistor Tr2then continues to operate as a unidirectional voltage source and thevoltage at the upper end of the capacitor C₁ follows the voltage Vt atthe emitter of the transistor Tr2.

When the wheel velocity voltage V drops rapidly at the time t₂, thevoltage Vw at transistor Tr1 drops with it. The voltage Vt at transistorTr2 attempts to drop with it. However, the capacitor is still chargedand the transistor Tr5 limits its discharge capability to a ratedetermined by the voltage g. This at t₂, the voltage Vt, instead offollowing the voltage V, drops less rapidly. Almost immediately, thevoltage V at the base of transistor Tr2 becomes lower than thedischarging voltage across the capacitor C₁ and cuts off the transistorTr2. At this point only the discharge current from the capacitor C₁passes through the emitter-collector path of transistor Tr5. Thedischarge proceeds linearly with a slope G corresponding to the voltagedrop. The discharge current from the capacitor C₁ is proportional to thevoltage g developed by the transistor Tr5 so as to generate the slope Gand reduce the voltage Vt linearly at the slope G. The slope G isdetermined by the voltage g applied from the set deceleration generator34 to the base of transistor Tr5.

The time t₂ defines when the wheel deceleration has become greater thanthe preset deceleration. Therefore, the time t₂ at which the wheels arein danger of being locked can be determined. This is so because thevoltage Vw at the emitter of transistor Tr1 is not inhibited fro fallingat the rapid rate of the voltage V. This is so because only thetransistor Tr6 forms a load for the emitter of transistor Tr1. Once theemitter of transistor Tr2 exhibits a voltage Vt, which departs from thevoltage Vw, the difference in voltage can be detected. This isaccomplished with high sensitivity by a differential amplifier composedof transistors Tr3 and Tr4 in comparator relationship. An emittercoupling resistor R₃ couples the transistors Tr3 and Tr4 in comparatorrelationship, and a load resistor R₁ forms an output terminal at thejunction of the collector of the transistor Tr4.

When the voltages Vw and Vt are applied to the bases of transistors Tr3and Tr4, and the voltage Vt exceeds the voltage Vw, the transistor Tr4conducts. A signal proportional to the voltage Vt then issues to thepressure modulator 26 which reduces the pressure of the braking fluid onthe basis of the signal received. A control circuit 36 connected to thecollector of transistor Tr2 may be used to correct the presetdeceleration of the set deceleration generator 34.

In operation the skid control system according to the invention causesthe generator 30 to charge the capacitor C₁ to a value proportional tothe wheel velocity as long as the wheel velocity change, that is to saythe wheel deceleration, is small enough so that the transistor Tr5 canhandle the discharge flow. Thus it charges as long as the wheeldeceleration is smaller than the preset deceleration. However, when thewheel deceleration becomes greater than the preset deceleration, that iswhen the voltage Vt drops faster than the capacitor C₁ is able todischarge through the transistor Tr5, the capacitor C₁ discharges onlyas fast as it can through the transistor Tr5. Thus the capacitor C₁ isdischarged without following the wheel velocity, and it is possible tocompare the two voltages. In this manner, the desired deceleration iscompared with an actual deceleration without the use of a wheel-velocitydifferentiation circuit.

In the embodiment of FIG. 2, the load circuit of transistor TR1constitutes the transistor Tr6, which exhibits the same characteristicsas the transistor Tr5. The same voltage g is applied to each. Accordingto another embodiment of the invention, the transistor is replaced by aresistor having a value that conforms to the performance range of thetransistor Tr6. Such a resistor R₅ is illustrated, as a detail in theenvironment of FIG. 2, in FIG. 5.

The transistors Tr1 and Tr2, which have the same characteristics, areselected to develop a differential voltage, they can then operate stablyeven if the ambient temperature is changed or power voltages varied.Under these circumstances, it is not necessary to provide a specifictemperature compensation circuit.

The circuit of FIG. 2 can be considered as forming a bridge composed offour arms. Two adjacent arms are formed by the transistors Tr1 and Tr6,and two adjacent arms are formed by the transistors Tr2 and Tr5. Thecapacitor C₁ also forms part of the arm which includes the transistorTr5. The comparator composed of transistors Tr3 and Tr4 then measuresthe voltage differences between the junctions at the pair of adjacentarms. In the embodiment of FIG. 5, the resistor R₅ constitutes the armadjacent to the transistor Tr1.

It will of course be understood that the systems of FIG. 2 and FIG. 5form a portion of a wheeled vehicle, such as an automobile, which thesystems represent. The control 36 may be a manually adjustable voltagesource or more complex mechanism. The same is true for the generator 34which may be varied as desired.

The preceding and following description is made with respect toautomobile brake systems, but may not be applicable to the brake systemsof other vehicles and is not limited to the automobile.

In FIG. 6, a brake pedal 1' of another automobile A embodying featuresof the invention actuates a master cylinder 2'. A control unit 3' of theskid control system forming a part of the automobile and this inventionelectrically regulates a pressure modulator 4'. The latter intervenesbetween the brake cylinder 2' and a wheel cylinder 5' so as to controlthe pressure in the wheel cylinder. The control unit 3' respondselectrically to the output of a velocity detector or generator 6' thatsenses the velocity of a wheel 7' which is controlled and stopped by thewheel cylinder 5'. The wheel 7' is understood here to represent one of anumber of wheels of the automobile A embodying features of thisinvention and whose speed is controlled by the brake system disclosedherein. Other wheels of the vehicle are understood to be controlled byother wheel cylinders responding to the master cylinder 2' and thepressure modulator 4'.

Under normal travel conditions, when the skid control system forming apart of the brake system is not required to operate, the pressure ofbrake fluid supplied from the master cylinder 2' conforms to the amountof physical effort applied to the brake pedal 1'. This pressure isapplied without change to the wheel cylinder 5' so as to produce acorresponding braking force. When emergency braking is applied on thepedal 1', there is danger of the wheel 7' locking due to a depletion ofthe frictional force between the wheel and the road surface. The controlunit 3' then responds to the wheel velocity detected by the wheelvelocity detector to issue a pressure reduction signal. By means of thelatter the pressure modulator 4' shuts off or reduces the flow of brakefluid from the master cylinder 2. It then regulates the pressure of thefluid in the wheel cylinder 5'. By preventing the wheels from locking inthis manner, the vehicle is brought to a halt within a minimum stoppingdistance.

In FIG. 7 changes in various coefficients of friction μ of wheels, suchas the wheel 7', are plotted along the ordinate axis of rectangularcoordinates, while the wheel slip ratio S is plotted on the abscissa.Thus FIG. 2 illustrates μ-S curves that vary in ways depending upon roadsurface conditions or vehicle speed or both. A curve a'₁ in FIG. 2illustrates a situation where the maximum coefficient of friction μ isobtained at a wheel slip ratio Sc. Other curves exhibit maximumcoefficients of friction at slip ratios less than Sc. The inventionpredetermines and utilizes a deceleration curve conforming to the roadsurface having the maximum coefficient of friction.

FIGS. 8a and 8b illustrate changes in the velocity V of the automobile Aand the velocity Vw of the wheel 6' with respect to time as the pressureP of the brake fluid increases linearly with respect to time. In FIGS.8a and 8b the time t is plotted along the abscissa. In FIG. 8a the wheelvelocity Vw' and the vehicle velocity are plotted along the sameordinate. In FIG. 8b the pressure P of the brake fluid is plotted alongthe ordinate. As the pressure P of the brake fluid increases, as aresult of pressure on the pedal 1, the velocity Vw' of the wheelgradually deviates from the vehicle velocity. This gradually increasesthe slip ratio. After the maximum coefficient of friction is reached atthe time t₂, the wheel velocity decreases, thus increasing thedeceleration rapidly. The deceleration of the wheel 7' at the time whenthe maximum coefficient of friction is reached, is shown by the slope αof the wheel velocity. Thus, if the wheel 7' is decelerated with thedeceleration α under the above described road surface conditions andvehicle velocity, the wheel will always decelerate with maximumcoefficient of friction. According to a feature of the invention, thepressure of the brake fluid is reduced with a slope corresponding to thedeceleration α by detecting the time t₂ when the wheel velocitydecreases rapidly. FIGS. 7, 8a and 8b are plotted without showing theeffects of modulator 4'.

FIG. 9 illustrates an embodiment of the control unit 3'. In FIG. 9 atransistor Tr1' of the control unit 3' receives a velocity dependentsignal from the wheel velocity detector 6' at its base. The varyingwheel velocity is biased to a level suitable for the base of transistorTr1' by a constant voltage network composed of a resistor R₅ ' thatenergizes a Zener diode. The transistor Tr1' is connected in commoncollector emitter follower configuration and produces a voltage Vvwproportional to the wheel velocity Vw across an emitter load resistor R₇'. The wheel velocity detector 6' also applied a voltage to the base ofa transistor Tr2'. The collector-emitter circuit of the latter charges acapacitor C₁₀ through a resistor R₁₀ '. The capacitor C₁₀ forms a wheelvelocity setting control.

The collector-emitter path of the transistor Tr2' charges the capacitorC₁₀ to a voltage Vvt' which normally corresponds to the wheel velocityVw'. Since the voltages applied to the bases of transistors Tr1' andTr2' are substantially the same, the emitter voltages tend to besubstantially the same. Thus the transistor Tr2' tends to charge thecapacitor C₁₀ to a voltage Vvt' comparable to the voltage Vvw'. As longas the voltage Vvw' exceeds the voltage Vvt', the capacitor will tend tocharge.

Transistors Tr3' , Tr4' and Tr5' form a differential amplifier. In thisdifferential amplifier the emitter collector path of the transistor Tr5'forms a coupling impedance. The one of the transistors Tr4' and Tr5'having a higher base voltage conducts. When the voltage Vvt' is higherthan the voltage Vvw', conduction of the transistor Tr4' develops avoltage drop in a collector resistor R₁ ' of the transistor Tr4' . Thisvoltage drop biases a p-n-p transistor into conduction and develops arising voltage p across a resistor R₂ '. The rising voltage drives atransistor Tr7' into conduction through a diode D₁. This increases thevoltage across the pressure modulator 4' and passes a pressure reductionsignal to the pressure modulator. Biasing resistors R₃ ' and R₄ 'normally bias the base and emitter of transistor Tr7' . The constantvoltage circuit composed of Zener diode ZD' and resistor R₅ ' forms aconstant current circuit with a resistor R₆ ' to furnish the baseemitter circuit in the emitter coupling resistor Tr5' with a constantsupply of base current.

The emitter collector circuit of a transistor Tr8 forms a discharge pathacross the capacitor C₁₀. This discharge path constitutes a correctingmeans. The discharge path of the emitter collector circuit of transistorTr8' is a direct path across the capacitor C₁₀. The conductivity of thepath is determined by a voltage g appearing across the capacitor C₂₀ andsupplied through a resistor R₈ ' to the base of the transistor Tr8' . Atransistor Tr9' charges the capacitor C₂₀. The latter transistor iscontrolled by the conduction of a p-n-p transistor Tr10' through acollector resistor R₁₂ ' and a coupling resistor R₁₃ '. The collector oftransistor Tr10' forms a voltage Vi' across the resistor R₁₂ '. Thus,when a voltage drop proportional to the charging current of thecapacitor C₁₀ appears at the base of the transistor Tr10' by virtue of acoupling resistor R₁₁ ', the base of the transistor Tr9' receives acurrent proportional to the charging current of the capacitor C₁₀through the resistor Tr10' . As a result, the capacitor C₂₀ is suppliedwith a charging current proportional to the charging capacitor. Thisreduces the voltage g which is adjusted by charging or discharging thecapacitor C₂₀.

A transistor Tr11' possesses a collector emitter circuit which forms adischarge path across the capacitor C₂₀. Two resistors, R₁₄ ' and R₁₇ ',form a voltage divider across the direct current supply of the batteryB. A biasing resistor R₁₅ ' applies the divided voltage d to bias thebase of transistor Tr11' . Conduction of the transistor Tr11' iscontrolled depending upon the magnitude of the voltage d at the junctionof resistors R₁₄ ' and R₁₇ '. Connected in parallel to the resistor R₁₄' is a series circuit composed of a transistor Tr12' and a diode D₃. Thetransistor Tr12' receives a base biasing voltage from the lower end ofthe resistor R₁₀ ', that is the collector voltage c of the transistorTr2' . A resistor R₁₆ ' is the collector biasing resistor of thetransistor Tr12' . Thus, the transistor Tr12' and the diode D₃ affectthe potential d formed by the voltage divider of resistors R₁₄ ' and R₁₇'. Also affecting the potential d is a diode D₄ connected between thepotential d and the voltage p. The diodes D₃ and D₄ form an off-circuit.When the voltage p at the collector of transistor Tr6' or the voltage cat the collector of transistor Tr12' is high relative to the negativeterminal of the source formed by the battery B, the voltage d will alsobecome high and drive the transistor Tr11 into non-conduction. On theother hand, when the voltage d exhibits the voltage formed exclusivelyby the voltage divider resistors R₁₄ ' and R₁₇ ', the transistor Tr11'conducts and the capacitor C₂₀ discharges. This increases the voltage gat one plate of the capacitor C₂₀.

The performance of the embodiment of the invention illustrated in FIG. 6and 9 may best be understood from reference to FIGS. 10a through 10g.FIG. 10a illustrates the changing wheel velocity Vw' and compares it tothe vehicle velocity V' as well as the velocity Vt' forming thedeceleration curve or slope, as each one of these varies with time. FIG.10b illustrates the pressure P' of the brake fluid in the wheel cylinder5' after the pressure has been modulated by the pressure modulator 4'.FIGS. 10c-10g illustrate the wave forms of the voltages p, c, d, Vi',and g in FIG. 9.

When the vehicle proceeds under normal travel conditions between thetimes t₀ and t₁, the wheel velocity Vw' is equal to the vehicle velocityV and almost constant. The capacitor C₁₀ charges up to a voltage equalto the wheel velocity Vw' so as to make the voltage Vvt' equal to thevoltage Vvw'. During this equality of the voltages the transistor Tr2'is substantially non-conductive. This keeps transistor Tr4'non-conductive, which in turn renders transistor Tr6' and Tr7'non-conductive or cut-off. With transistor Tr7' cut-off, the pressuremodulator 64 receives no pressure modulation signal. Simultaneously, thetransistor Tr10' receives its base voltage from the resistor R₁₀ ' whichcarries substantially no current. Thus the transistor Tr10' remainscut-off and serves to cut off the transistor Tr9'. Transistor Tr12' alsoreceives its base voltage from the resistor R₁₀ ' and similarly does notconduct. However, the voltage divider composed of resistors R₁₄ ' andR₁₇ ' furnishes a forward biasing potential to the base of transistorTr11' and causes it to conduct. Thus the emitter collector circuit ofthe transistor Tr11 shortcircuits the capacitor C₂₀ and drives thevoltage g at the lower plate of the capacitor C₂₀ to its maximum, thatis to it most positive potential. This positive potential drives thetransistor Tr8' into conduction and serves to discharge the capacitorC₁₀. However, the transistor Tr2' conducts enough to continouslyrecharge the capacitor C₁₀. However, this conduction is not sufficientto turn on the transistor Tr10', that is to render the transistor Tr10'conductive.

In this manner the circuit maintains the voltage Vvt equal to thevoltage Vvw'.

During the period between the times t₁ and t₂, the brake pedal 1 isapplied for an emergency stop. The wheel velocity Vw' startsdecelerating because of the increase in the pressure of the brake fluidin the master cylinder 2' and the wheel cylinder 5'. However, the wheeldeceleration is still less than the preset deceleration established bythe rate at which the transistor Tr8 discharges the capacitor C₁₀. Thatis to say, the transistor Tr8' discharges the capacitor C₁₀ faster thanthe voltage Vvt tends to drop. Thus the voltage Vvt' can drop togetherwith the voltage Vvw'. In this manner these two voltages remain equal.Thus during the period between the times t₁, and t₂, each of thetransistors retain their same operating conditions as during the timeperiod between times t₀ and t₁. The current flow caused by thetransistor Tr2 through the resistor R₁₀ ' causes an insufficient voltagedrop to turn on the transistor Tr10'. The condition of the transistorsTr6', Tr12', Tr10' and Tr9' are such as to be analyzable as eitherconductive or non-conductive. Thus the voltages p, c and Vi may beconsidered as logic signals which are either "high" or "low", or "1" or"0".

It is possible for the wheel velocity Vw' to drop more rapidly than thedeceleration preset by the voltage g. This happens when the wheelvelocity drops so rapidly that it drives the voltage Vvt' down fasterthan the transistor Tr8' is capable of discharging the capacitor C₁₀.The rate at which the transistor Tr8' discharges the capacitor C₁₀ isdetermined by the voltage g. This is because the transistor Tr8' issubstantially a current amplifier whose emitter collector circuitcarries current in proportion to the voltage g. At some point thevoltage Vvt' may attempt, as a result of rapid deceleration, to dropfaster than this preset rate. This occurs in FIG. 10 at the time t₂ whenthe coefficient of friction between the wheel and road surface reaches amaximum and the wheel velocity Vw' decreases rapidly. Since the voltageacross the capacitor C₁₀ cannot discharge as fast as the voltage Vvttends to drive it down, the voltage Vvt'is forced to remain at a higherpotential, that is a more positive potential than the voltage Vvw' . Theunbalance causes the transistor Tr4' to conduct. This drives thetransistor Tr6' on, i.e. into conduction and causes the collectorvoltage p of the transistor Tr6' to assume a logic 1. The voltage pdrives the transistor Tr7' into conduction through the diode D₁. Thecollector voltage of the transistor Tr7' signals the pressure modulator4' to reduce the pressure P of the brake fluid. At the same time thehigh potential, that is the positive potential p, is applied through thediode D₄ ' to the resistor R₁₅ ' at the base of transistor Tr11'. Thisrenders the transistor Tr11 non-conductive.

The signal to the pressure modulator to reduce the pressure P'of thebrake fluid reduces the braking force. The wheel velocity thenincreases, in response to this reduction of pressure, to approach thevehicle velocity. At the time t₃ it exceeds the deceleration curveestablished by the voltage g and the transistor Tr8'. This decelerationcurve decreases with a fixed slope determined by the voltage g from thetime t₂. This is because the transistor Tr8' discharges the capacitorC₁₀ at a constant rate established by the voltage g. The voltage Vvteventually reaches the voltage Vvw at the time t₃. This eliminates theunbalance at transistors Tr3' and Tr4', and transistor Tr4' stopsconducting. This turns off transistor Tr6' to change the logic level ofvoltage p to 0 and turn off transistor Tr7'. This non-conduction oftransistor Tr7' eliminates the pressure reduction signal in the pressuremodulator 4' and allows the modulator to increase the pressure P' asshown in FIG. 10b by the solid line.

However, at this time before the wheel velocity stabilizes or reversesin response to the changing pressure, it still increases and tends toraise the value of the voltage Vvt above the charge level of thecapacitor. This causes the transistor Tr2' to conduct heavily enough tocharge the capacitor C₁₀ and to produce conduction of the transistorTr2' and thereby cause a voltage drop across the resistor R₁₀ '. Thisapplies forward potentials to the bases of transistors Tr10' and Tr12'.They thus conduct and cause the voltages c and Vi' to exhibit a logic 1.As a result the diode D₃ applies the voltage c to the base of transistorTr11' to overcome its normal biasing potential and drive it intonon-conduction. The voltage Vi' responds to the conduction of thetransistor Tr10' by driving the transistor Tr9' into conduction. Thiscauses capacitor C₂₀ to charge and decrease the voltage g. The latterreduction reduces the conduction of the emitter-collector current pathof transistor Tr8' and allows the capacitor C₁₀ to charge gradually. Asa result the voltage Vvt is corrected at a rate that follows theincrease in the wheel velocity Vw'. More specifically, the voltage Vvt'changes at a rate corresponding to the decrease in the voltage g.

Eventually, the increased pressure P' of the brake fluid in the brakecylinder 5 stops the increase in the velocity Vw' so that it reaches amaximum at a time t₄. The wheel velocity Vw' starts to decrease againdue to the increased braking force. At this time the transistor Tr2'stops charging the capacitor C₁₀ and in effect becomes essentiallynon-conductive. This turns off the transistor Tr10' as well as thetransistor Tr9' and interrupts charging of the capacitor C₂₀. It alsocuts off the transistor Tr12', that is it renders the transistornon-conductive and allows the voltage divider R₁₄ ' and R₁₇ ' to biasthe transistor Tr11'into conduction. The latter then serves to act as aconstant current discharge path for the capacitor C₂₀. This increasesthe voltage g almost linearly. Consequently, the conductivity of thetransistor Tr8' increases so as to increase th discharge rate of thecapacitor C₁₀. The voltage Vvt'is then corrected so that its slopeincreases in proportion to the decrease of the wheel velocity Vw.Eventually, as the pressure in the cylinder 5' decreases the velocity ofthe wheel 7', the deceleration of the wheel exceeds the new decelerationdetermined by the voltage drop in the capacitor C₁₀ and by the newvoltage g. This occurs at the time t₅. When the wheel exceeds the slipratio at which the coefficient of friction is maximum, the wheelvelocity setting means furnishes a pressure reduction signal to thepressure regulator. That is to say as the drop in wheel velocity causesthe voltage Vvt' to attempt to drop fashter than the transistor Tr8' iscapable of discharging the capacitor C₁₀ the transistor Tr4' conducts.The transistor Tr7' is then caused to furnish a pressure reductionsignal to the pressure modulator 4' by the conduction of transistorTr6'.

As before the decrease in the pressure of the brake fluid in the wheelcylinder 5' decreased deceleration and causes a rise in the wheelvelocity until the voltage Vvw' again reaches the voltage Vvtestablished across the capacitor C₁ '. At that point the signal to thepressure modulator 4' ends and the high brake pressure applied by themodulator again appears in the wheel cylinder 5'. This occurs at thetime t₆. Between the times t₆ and t₇ the transistor Tr2' exhibits anemitter voltage which is attempting to rise faster than the voltage ofthe capacitor C₁₀ and therefore conducts heavily. The heavy conductionresults in a reduction of the voltage g by virtue of the chargingproduced at the more negative plate of the capacitor C₂₀ by thetransistor Tr9'. A second adjustment of the voltage g occurs during theperiod between the times t₇ and t₈. This occurs because the effect offluid pressure in the wheel cylinder 5' causes the wheel 7' again toreach a maximum and start to slow down. The falling velocity sensed bythe generator 6' appears as a falling potential Vvt' at the emitter oftransistor Tr2' and across the capacitor C₁₀. This ends the charging ofthe capacitor C₁₀ by the transistor Tr2' and initiates resumption ofconduction by the transistor Tr11'. The latter discharges capacitor C₂₀and raises the level of the voltage g until the time t₈. At that momentthe voltage Vvt, because of the deceleration actuated by the fullpressure of the pedal 1' , is attempting to decrease faster than thetransistor Tr8' is capable of dischargingthe capacitor C₁₀. Theunbalance in the differential amplifier composed of transistors Tr3' andTr4' causes conduction of the transistor Tr6' as well as Tr7' to producea pressure reduction signal to the pressure modulator 4'. Conduction ofthe transistor Tr6' again produces a more positive signal at thejunction of the resistors R₁₄ ' and R₁₇ ' so as to cut off theconduction of the transistor Tr11' and thereby stop discharge of thecapacitor C₂₀. The cycle is again repeated between times t₈ and t₉ andtimes t₉ and t₁₀.

The above-mentioned adjustments or corrections of the voltage g producesa mean value g₁ which conforms to the actual road surface conditions. Inthis way the time during which the wheel velocity Vw' is decreased withan optimum deceleration, that is with a deceleration conforming to theroad surface conditions, is extended. The velocity decrease beyond theslip ratio, at which the coefficient of friction is maximum, isdecreased. Consequently, the vehicle, namely the automobile A is broughtto a halt with its brakes near an average slip ratio at which thecoefficient of friction is maximum.

When the wheel velocity Vw' is restored and exceeds the decelerationcurve Vt', that is when the voltage Vvw exceeds the voltage Vvt', thetransistor Tr7' is rendered non-conductive and an increased pressuresignal is passed to the pressure modulator 4'. It is possible to hastenthe restoration of the wheel velocity by maintaining the pressurereduction signal beyond a time such as t₃. This is done, as shown inFIG. 11, by applying the positive voltage c that turns off thetransistor TR11', to the base of transistor Tr7' by means of a diode D₂.This voltage then turns on the diode Tr7' and continues the pressurereduction signal. The circuit in FIG. 11 is an abbreviated illustrationof the circuit in FIG. 9, but with the addition of a diode D₂. Onlythose portions of the circuit 3' in FIG. 9, which are necessary forillustrating the connections of the diode D₂, are shown. In FIG. 11 thevoltage c is applied to the base of the transistor Tr7' which thenfurnishes a pressure reduction signal to the pressure modulator 4', evenwhen the voltage c exhibits a logic value 1. Thus the pressure of thebrake fluid changes in FIG. 11 according to the path shown by the dottedline in FIG. 10b. An increased pressure signal is applied to thepressure modulator 4' only when the voltages p and c have a logic value0.

Another method of accomplishing a similar purpose is to include aretaining value in the pressure modulator 4'. If this is done, thepressure P of the brake fluid is maintained at a constant value betweenthe times t₃ and t₄ and times t₆ and t₇ as shown by the dot-dash linesin FIG. 10b.

FIG. 12 illustrates another circuit which may be used as the controlunit in FIG. 6 and which embodies features of the invention. In FIG. 12the control unit 3' is in part identical to the control unit 3' shown inFIG. 9 The main difference resides in the elimination of transistorsTr11', Tr12', the resistors R₁₁ ' , R₁₄ ' , R₁₅ ' , R₁₆ ' and R₁₇ 'which form the discharge circuit of the capacitor C₂₀, and the diodes D₃and D₄ of the off-circuit which form the voltages c and d. Instead, aresistor R₁₈ ' connected across the capacitor C₂₀ forms the dischargecircuit of the capacitor. Thus the capacitor C₂₀ is discharged by meansof the resistor R₁₈ ' when the transistor Tr9' is not conducting. Thecapacitor C₂₀ thus performs a first adjusting step that increases thevoltage g along the curve g' of FIG. 5g.

This embodiment of the skid control system operates in a manner similarto the skid control system using the control unit 3' of FIG. 9. However,here instead of the capacitor C₂₀ being discharged by an alternatelyconductive and non-conductive transistor Tr11' , the capacitor C₂₀ isdischarged continuously by a resistor R₁₈ '. The capacitor C₂₀ ischarged as in FIG. 4 when the transistor Tr2' conducts heavily, therebyturning on the transistor Tr10' and the transistor Tr9'.

A pressure reduction signal in FIG. 12 is generated in the same manneras in FIG. 9. That is when the voltage Vvt' attempts to drop faster thanthe transistor Tr8' is capable of discharging the capacitor C₁₀.conduction of transistor Tr4' turns on the transistor Tr6'which in turnturns on the transistor Tr7'.

The embodiment of the invention as illustrated in part in FIG. 7 may bemodified to change the pressure P' of the brake fluid so as to followthe path shown by the dotted line in FIG. 10b. This speeds up therestoration of the wheel velocity Vw'. This is accomplished by modifyingthe circuit of FIG. 12 with the addition of the diode D₂ as shown inFIG. 13. The diode D₂ applies the voltage Vi, appearing at the collectorof transistor Tr10', across the base-biasing resistor R₃ ' of thetransistor Tr7'. This results in application of a pressure reductionsignal to the pressure modulator 4' even when the voltage p exhibits alogic value 0, as long as Vi exhibits 1.

according to yet another embodiment of the invention the curve shown bydot-dash lines in FIG. 10b is obtained as shown in FIG. 14. Here thevoltage Vi' at the collector of transistor Tr10' is applied to the baseof a transistor Tr7a whose base and collector are biased by resistorsR_(3a) and R_(4a). The remainder of the circuit in FIG. 14 correspondsidentically to the circuit in FIG. 12. The emitter of transistor Tr7aenergizes a retaining value 60 in the pressure modulator 4', which isillustrated in more detail in FIG. 10. The voltage Vi, which issubstantially 0 until the time t₃ rises at that time to turn on thetransistor Tr7a. This operates the retaining valve 60.

In FIG. 15 the pressure modulator 4 includes a pressure modulating unit40, a change-over valve 50 and a retaining valve 60. The pressuremodulating unit 40 is composed of a cylinder 41, a diaphragm 42 dividingthe larger bore portion of the cylinder 41, a piston 43 projecting fromthe diaphragm 42, a seal 44, a check ball 45 and springs 46 and 48. Thecylinder 41 is divided by the diaphragm 42 into a working-chamber 41a,and a pressure-receiving chamber 41b. A modulating chamber 41c receivesfluid from an introduction chamber 41d. The change-over valve 50includes a cylinder 51, a valve body 52 and an electromagnetic coil 53.The signal developed by a transistor Tr7' of the control unit 3'normally energizes the coil 53. The latter, when excited, holds thevalve body 52 in the position illustrated. Atmospheric pressure isthereby introduced from a connecting port 51c through a port 51a and theretaining valve 60 into an entrance port 47a that leads to the workingchamber 41a of the pressure modulating unit 40. When the coil 53 isdeenergized, a spring not shown moves the valve body 52 downwards so asto cause communication between the connection port 51a and theconnection port 51b. This results in fluid connection of both theworking chamber 41a and pressure receiving chamber 41b to a pressurereceiving source 70. In the pressure modulating unit, brake fluid fromthe master cylinder is directed through a port 49 into the introductionchamber 41d and through the modulating chamber 41c. The brake fluid thenpasses to the wheel cylinder 5 through a connection port 47c. When thepressure modulator 4 is used with control units 3', as illustrated inFIGS. 9, 11, 12 and 13, the retaining valve 60 substantially behaves asan open port or may be eliminated entirely. The operation of thepressure modulator 4, when operating with the control units 3 of FIGS.9, 11, 12 and 13, has its coil 53 deenergized under normal travelconditions. Under these circumstances, both chambers 41a and 41b are inthe pressure-receiving state, that is, they are under vacuum pressurefrom source 70. Therefore, the spring 48 presses the diaphragm 42 to theright and the tip of the piston 43 pushes a check ball 45 open to theright. Consequently, brake fluid from the master cylinder communicatesthrough the chambers 41c and 41dto the wheel cylinder. This appliesbrake fluid to the wheel cylinder with the pressure conforming to theamount of physical effort applied to the brake pedal 1'. On the otherhand, a pressure reduction signal issued from the control unit 3'excites the magnetic coil 53. This lifts the valve body or piston 52 tothe position shown, and atmospheric pressure is supplied into thework-chamber 41a. The diaphragm 42 and piston 43 move to the left toallow a spring 46 to close the check ball 45. This shuts off the flow ofbrake fluid to the wheel cylinder. The movement of the piston 43 to theleft increases the volume of the modulating chamber 41c. This decreasesthe pressure of brake fluid to the wheel cylinder 5'. Termination of thepressure reduction signal at this time deenergizes the electromagneticcoil 53 and causes the piston 43 to move to the right into a range wherethe brake line is still closed by the check ball 45. As a result, thevolume of the modulating chamber 41c is decreased, and the pressure ofbrake fluid to the wheel cylinder 5' is increased.

At the next pressure reduction signal, electromagnetic coil is excitedand the piston 43 moves to the left within a range where the brake lineis closed by the check ball 45. This increases the volume of themodulating chamber 41c and decreases the pressure of the brake fluid. Asdescribed, the pressure modulating unit increases or decreases thepressure of brake fluid through movement of the piston 43 within therange where the brake line remains closed by the check ball 45.

In the retaining valve 60, a cylinder 61 receives a valve body 62 whichis moved by an electromagnetic coil 63. Connection ports 61a and 61bconnect the retaining valve 60 to the change-over valve 50 and theworking chamber 41a of the pressure modulating unit 40. The retainingvalve 60 makes it possible for the pressure to follow the pathillustrated in FIG. 10b by the horizontal dot-dash lines between thetimes t₃ and t₄ and between the times t₆ and t₇. According to oneembodiment of the invention, this is accomplished as shown in FIG. 16.FIG. 11 illustrates a portion of the control unit 3' which is otherwisecomparable to the control unit 3' of FIG. 9. However, here the voltage cacross the resistor R₁₆ is applied to the relay coil 63.

According to the embodiment of the invention illustrated in FIG. 14, acurrent is applied to the electromagnetic coil 63 through the transistorTr7a. Here it is the voltage Vi', which is applied to the base of thetransistor Tr7a. In this manner, the electromagnetic coil 63 is excitedand the valve body 62 moved upwardly. This shuts off the supply ofatmospheric pressure into the working chamber 41a and stops the movementof the diaphragm 42. In this way, the pressure of the brake fluid ismaintained at a constant value for a fixed period of time.

In the skid control system according to this invention the wheelvelocity detector 6' detects the wheel velocity. The wheel velocitysetting means of the control unit 3' compares the wheel deceleration andthe preset deceleration by charging or discharging the capacitor C₁₀.This produces or stops a pressure reduction signal. The first and secondcorrecting or adjusting means in the capacitor adjust the decelerationcurve in accordance with the wheel deceleration by regulating thevoltage g. The pressure modulator 4' regulates the pressure of thebraking fluid depending upon the presence of a pressure reductionsignal. Consequently, the wheels are slowed or receive a braking forceso that the average slip ratio corresponds to the point where thecoefficient of friction is maximum. The vehicle is thus stopped in thesmallest stopping distance dependng upon the road surface conditions.This is done without the wheels locking. According to still anotherembodiment of the skid control system according to the invention, theelectronic circuits shown in the embodiments are replaced withmechanical hydraulic or pneumatic elements. For example, when thecircuits are replaced with mechanical elements, the capacitor C₁₀ isreplaced with a fly wheel. The transistor Tr8', that discharges thecapacitor C₁₀ is replaced with a friction brake for the deceleration ofthe fly wheel. The transistor Tr2', which charges the capacitor C₁₀, isreplaced with one-way clutch for the acceleration of the fly wheel. Thedifferential amplifier is then replaced by a governor mechanism.

As disclosed herein, the discharge circuit for the capacitor C₂₀, namelycomposed of the transistor Tr11', is biased normally by the voltagedivider composed of resistors R₁₄ ' and R₁₇ ' into a conductivecondition so as normally to discharge the capacitor C₂₀. Thiseffectively raises the value of g to a nominally high voltage. Thispermits a comparatively high rate of velocity decrease. The invention isbased upon the recognition that the decrease in wheel velocity becomesmeasurably greater when the slip ratio reaches the point of maximumcoefficient of friction. At this point, the system reduces the brakingforce until the wheel velocity corresponds to the wheel velocity whichis established by the desired velocity decrease. The circuits thendecrease the value of the voltage g, that is the desired deceleration orrate of decrease on the basis of how long the wheel velocity takes toreach a peak in response to an increased brake pressure. This change ismodified on the basis of the amount of time it takes for the slip ratioto reach the point of maximum coefficient of friction.

The pressure modulator illustrated in FIG. 15 is only an example of apressure modulator suitable for use in the system of FIG. 6. Otherpressure modulators may be used. For example, any valve that slowlydiverts the pedal pressure during the pressure reduction signal, butthat gradually reapplies the pedal pressure to build up toward the pedalpressure, may be used.

In the pressure modulator of FIG. 15, the piston 43 does not unseat theball 44 in the intervals between pressure reduction signals, such asthose between times t and t₁₀ in FIGS. 10a through 10g. This is becausea new pressure reduction signal occurs before the piston has a chance toreach the ball 44.

While embodiments of the invention have been described in detail, itwill be obvious to those skilled in the art that the invention may beembedded otherwise without departing from its spirit and scope.

What is claimed is:
 1. For a vehicle with wheels and a brake, a brakecontrol system for regulating the braking effect of the brake,comprising:detector means for producing a variable detector signalcorresponding to the speed of a wheel, storage means coupled to saiddectector means for storing the detector signal and for forming areference signal representing a momentary reference speed having achangeable maximum variability corresponding to a changeable desireddeceleration, brake modifying means coupled to said detector means andsaid storage means for comparing the reference signal with saiddectector signal and for producing a brake relieving signal when thedetector signal and said reference signal have a given relationindicating that the wheel speed has decelerated faster than the givendeceleration, whereby the brake relieving signal decreases thedeceleration of the wheel and may cause the wheel speed to be greaterthan the momentary reference speed, said dectector means responding tochanges in the wheel speed and said storage means responding to saiddetector means when the wheel speed is greater than the momentaryreference speed to produce given changes in the reference signal,coupling means coupled to the storage means and said brake modifyingmeans for causing the brake modifying means to continue the brakerelieving signal during the given changes in the reference signal saidcoupling means including a transistor coupled to said storage means forresponding to the given changes in the storage means and a diodecoupling said transistor to said brake modifying means.
 2. A device asin claim 1, wherein said braking modifying means includes a valve, coilcoupled to said storage means and a valve to produce the brake relievingsignal.
 3. A device as in claim 1, wherein said storage means include acapacitor, a transistor connected across said capacitor for dischargingsaid capacitor, a second transistor and a resistor serially connectedwith said capacitor for charging said capacitor, the current throughsaid second transistor and said resistor being indicative of the givenchanges.
 4. A device as in claim 1, where said brake modifying meansincludes a transistor amplifier and a hydraulic pressure modulator
 5. Adevice as in claim 4, wherein said coupling means includes a transistorcoupled to said comparator means for responding to the given changes inthe comparator means and a diode coupling said transistor to said brakemodifying means said diode being connected to said amplifier.
 6. For avehicle with wheels and a brake, a brake control system for regulatingthe braking effect of the brake, comprising: detector means forproducing a variable detector signal corresponding to the speed of awheel,storage means coupled to said detector means for storing thedetector signal and for forming a reference signal representing amomentary reference speed having a changeable maximum variabilitycorresponding to a changeable desired deceleration, brake modifyingmeans coupled to said detector means and said storage means forcomparing the reference signal with said detector signal for producing abrake relieving signal when the dector signal and said reference signalhave a given relation indicating that the wheel speed has deceleratedfaster than the given deceleration, whereby the brake relieving signaldecreases the deceleration of the wheel and may cause the wheel speed tobe greater than the momentary reference speed, said detector meansresponding to changes in the wheel speed and said storage meansresponding to said detector means when the wheel speed is greater thanthe momentary reference speed to produce given changes in the referencesignal, control means coupled to the storage means and responsive to thegiven changes for storing a control signal corresponding to the givenchanges and for varying the maximum variability of the reference signalin response to the given changes and coupling means coupled to saidstorage means and said brake modifying means for constraining the brakerelieving signal to continue during the given changes in the referencesignal.
 7. A device as in claim 6, wherein said coupling means includesa transistor coupled to said storage means for responding to the givenchanges in the storage means and a diode coupling said transistor tosaid brake modifying means.
 8. A device as in claim 6, wherein saidcontrol means includes a valve coil coupled to said storage means and tosaid brake modifying means for actuating said brake modifying means. 9.A device as in claim 6 wherein said storage means include a capacitor, atransistor connected across said capacitor for discharging saidcapacitor, a second transistor and a resistor serially connected withsaid capacitor for charging said capacitor, the current through saidsecond transistor and said resistor being indicative of the givenchanges said first transistor being responsive to said control means.10. A device as in claim 6, wherein said control means includes a valvecoil coupled to said storage means and to said brake modifying means foractuating said brake modifying means said hydraulic pressure modulatorincluding a valve, said coil being coupled to said valve.